U.S. patent application number 11/530644 was filed with the patent office on 2007-01-11 for thin film transistor in which fluctuations in current flowing therethrough are suppressed, and image display apparatus.
Invention is credited to Koichi Miwa, Mitsuo Morooka, Shinya Ono, Takatoshi Tsujimura.
Application Number | 20070007528 11/530644 |
Document ID | / |
Family ID | 33528549 |
Filed Date | 2007-01-11 |
United States Patent
Application |
20070007528 |
Kind Code |
A1 |
Tsujimura; Takatoshi ; et
al. |
January 11, 2007 |
THIN FILM TRANSISTOR IN WHICH FLUCTUATIONS IN CURRENT FLOWING
THERETHROUGH ARE SUPPRESSED, AND IMAGE DISPLAY APPARATUS
Abstract
A thin film transistor according to the present invention
includes a gate electrode, a semiconductor layer having a channel
forming region arranged on the gate electrode and an impurity
region arranged on a part of the channel forming region, source and
drain electrodes electrically connected to the impurity region, and
a gate insulating film that electrically insulates the gate
electrode and the semiconductor layer, wherein the distance between
the upper end of the gate electrode and the upper end of the
impurity region is larger than the distance between the upper end
of the gate electrode and the upper end of the channel forming
region.
Inventors: |
Tsujimura; Takatoshi;
(Kanagawa, JP) ; Ono; Shinya; (Kanagawa, JP)
; Morooka; Mitsuo; (Kanagawa, JP) ; Miwa;
Koichi; (Kanagawa, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
33528549 |
Appl. No.: |
11/530644 |
Filed: |
September 11, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10843337 |
May 12, 2004 |
|
|
|
11530644 |
Sep 11, 2006 |
|
|
|
Current U.S.
Class: |
257/59 ; 257/72;
257/E29.137; 257/E29.28; 257/E29.291; 257/E33.064 |
Current CPC
Class: |
H01L 29/42384 20130101;
H01L 29/78669 20130101; H01L 29/78696 20130101; H01L 29/78609
20130101 |
Class at
Publication: |
257/059 ;
257/072; 257/E33.064 |
International
Class: |
H01L 29/04 20060101
H01L029/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2003 |
JP |
2003-139476 |
Claims
1. An active-matrix-type image display apparatus comprising: a
plurality of pixel circuits, each of which includes an organic
light emitting element emitting light of brightness corresponding
to a current applied; a driver element electrically connected to
the organic light emitting element to control light emission of the
organic light emitting element, having a first thin film
transistor; a switching element controlling a voltage to the first
thin film transistor, having a second thin film transistor, wherein
the first thin film transistor and the second thin film transistor
includes a gate electrode; a dielectric layer on the gate
electrode; a semiconductor layer including a channel region and an
impurity region on a part of the channel region, formed on the
dielectric layer; a source electrode formed on the impurity region;
and a drain electrode formed on the impurity region, arranged to be
separated from the source electrode, wherein at least one of the
source electrode and the drain electrode has an overlapped region
with the gate electrode, the overlapped region being in contact
with the impurity region, a first distance from an upper surface of
the gate electrode to an upper surface of the impurity region is
longer than a second distance from an upper surface of the gate
electrode to an upper surface of the channel forming region, and a
first thickness of the semiconductor layer in a region under at
least one of the source electrode and the drain electrode is
thicker than a second thickness of the semiconductor layer in a
region between the source electrode and the drain electrode;
wherein the drain electrode of the switching element or the source
electrode of the switching element is electrically connected to the
organic light emitting element; wherein the drain electrode of the
switching element or the source electrode of the switching element
is electrically connected to the gate electrode of the driver
element.
2. The active-matrix-type image display apparatus according to
claim 1, wherein the semiconductor layer is an amorphous silicon
layer.
3. The active-matrix-type image display apparatus according to
claim 1, wherein the first thickness of the semiconductor layer is
in a range of 170 nanometers and 230 nanometers, and the second
thickness of the semiconductor layer is in a range of 85 nanometers
and 115 nanometers.
4. The active-matrix-type image display apparatus according to
claim 1, wherein the switching element controls a short-circuiting
between the gate electrode and the drain electrode or the source
electrode or the source electrode of the driver element.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of co-pending application
Ser. No. 10/843,337, filed on May 12, 2004, for which priority is
claimed under 35 U.S.C. .sctn. 120. Application Ser. No. 10/843,337
claims priority under 35 U.S.C. .sctn. 119(a) on Patent Application
No. 2003-139476 filed in Japan on May 16, 2003. The entire contents
of both are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1) Field of the Invention
[0003] The present invention relates to a thin film transistor used
for a pixel circuit having a current-controlled light emitting
element, and an image display apparatus.
[0004] 2) Description of the Related Art
[0005] An organic electro-luminescence (EL) display apparatus using
an organic light-emitting-diode (LED) that emits light autonomously
is getting an attention as a next generation image display
apparatus, because it does not require a back light that is
necessary in a liquid crystal display (LCD) apparatus, which makes
it most suitable for reducing thickness of the apparatus, and does
not have any limitation in the angle of visibility. Unlike the
liquid crystal display apparatus in which a liquid crystal cell is
controlled by a voltage, the organic LED used for the organic EL
display apparatus has a mechanism that the brightness of each light
emitting element is controlled by a current.
[0006] In the organic EL display apparatus, a simple (passive)
matrix type and an active matrix type can be employed as a drive
system. The former has a simple configuration, but has a problem of
realization of a large and high definition display. Therefore, a
research and development is focused on an active-matrix-type image
display apparatus that controls current flowing in a light emitting
element in a pixel by a driver element such as a thin film
transistor (TFT) provided in the pixel.
[0007] The pixel circuit included in the active-matrix-type image
display apparatus includes a driver element that is serially
connected to an organic light emitting diode (LED), and controls
light emission of the organic LED and the brightness of the light
emission, and a switching element that is connected to the driver
element and controls the operation for transmitting voltage
supplied from a data line to the inside of the pixel circuit
(hereinafter, "write") (see for example, Japanese Patent
Application Laid-Open No. H8-234683 (page 10 and FIG. 1)). The
voltage written into the pixel under the control of the switching
element is applied to a gate electrode of the driver element, and
the driver element controls the current flowing into the organic
LED, regulating the current flowing through the driver element by
the voltage applied to the gate electrode.
[0008] However, when the TFT using amorphous silicon for a
semiconductor layer is operated for long time, the threshold
voltage fluctuates, as shown in FIG. 10, and hence the current
flowing through the TFT also fluctuates, causing a problem of
degradation of image quality. FIG. 11 is a graph of voltage-current
characteristic of a TFT with the current started to flow and the
TFT with the current flowed for long time. As shown by curve
I.sub.3, the threshold voltage of the initial TFT when the current
starts to flow is V.sub.th. However, the threshold voltage of the
TFT in which the current has flowed for long time changes, as shown
by curve I.sub.4, from V.sub.th to V.sub.th' in the positive
direction. At this time, even when the gate-source voltage has the
same value V.sub.D, the drain current flowing through the TFT
changes from I.sub.d1 to I.sub.d2 (<I.sub.d1). When the
threshold voltage of the TFT used for the driver element
fluctuates, even when the voltage supplied into the pixel circuit
is the same, the current flowing through the driver element
fluctuates, and hence the current flowing through the organic LED
also fluctuates. Therefore, the emission brightness of the organic
LED becomes nonuniform, thereby deteriorating the image quality of
the image display apparatus.
SUMMARY OF THE INVENTION
[0009] The thin film transistor according to one aspect of the
present invention includes a gate electrode arranged on a
substrate; a semiconductor layer including a channel forming region
arranged on the gate electrode, and an impurity region arranged on
a part of the channel forming region; a source electrode and a
drain electrode electrically connected to the impurity region and
arranged to be separated from each other; and a gate insulating
film that is provided between the gate electrode and the
semiconductor layer, and electrically insulates the gate electrode
and the semiconductor layer. A distance between an upper end of the
gate electrode and an upper end of the impurity region is longer
than a distance between an upper end of the gate electrode and an
upper end of the channel forming region.
[0010] The active-matrix-type image display apparatus according to
another aspect of the present invention includes a light emitting
element that emits light of brightness corresponding to a current
applied; a driver element that is connected to the light emitting
element in series to control light emission of the light emitting
element, including a thin film transistor; and a switching element
that controls a voltage to a gate electrode of the thin film
transistor. The thin film transistor includes the gate electrode
arranged on a substrate; a semiconductor layer including a channel
forming region arranged on the gate electrode and an impurity
region arranged on a part of the channel forming region; a source
electrode and a drain electrode electrically connected to the
impurity region and arranged to be separated from each other; and a
gate insulating film that is provided between the gate electrode
and the semiconductor layer, and electrically insulates the gate
electrode and the semiconductor layer. A distance between an upper
end of the gate electrode and an upper end of the impurity region
is longer than a distance between an upper end of the gate
electrode and an upper end of the channel forming region.
[0011] The active matrix type image display apparatus according to
still another aspect of the present invention includes a light
emitting element that emits light of brightness corresponding to a
current applied; a driver element that is connected to the light
emitting element in series to control light emission of the light
emitting element; a current determining element that controls a
current flowing through the driver element by passing the current
corresponding to an applied voltage, including a thin film
transistor; and a capacitor that converts the current flowing
through the current determining element into voltage and holds the
voltage as gate-source voltage of the driver element. The thin film
transistor includes the gate electrode arranged on a substrate; a
semiconductor layer including a channel forming region arranged on
the gate electrode and an impurity region arranged on a part of the
channel forming region; a source electrode and a drain electrode
electrically connected to the impurity region and arranged to be
separated from each other; and a gate insulating film that is
provided between the gate electrode and the semiconductor layer,
and electrically insulates the gate electrode and the semiconductor
layer. A distance between an upper end of the gate electrode and an
upper end of the impurity region is longer than a distance between
an upper end of the gate electrode and an upper end of the channel
forming region.
[0012] The other objects, features, and advantages of the present
invention are specifically set forth in or will become apparent
from the following detailed description of the invention when read
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a cross section of a TFT according to a first
embodiment of the present invention;
[0014] FIG. 2 is a graph of a result of measurement of fluctuations
in threshold voltage of the TFT according to the first
embodiment;
[0015] FIG. 3A is a graph of a result of measurement of
fluctuations in the threshold voltage when the TFT according to the
first embodiment operates in a linear region;
[0016] FIG. 3B is a graph of a result of measurement of
fluctuations in the threshold voltage when the TFT according to the
first embodiment operates in a saturated region;
[0017] FIG. 4 is a graph of fluctuation characteristics of the
threshold voltage of TFTs having different channel length in the
TFT according to the first embodiment;
[0018] FIG. 5 is another example of cross section of the TFT
according to the first embodiment;
[0019] FIGS. 6A to 6D are schematics for illustrating a method of
manufacturing the TFT according to the first embodiment;
[0020] FIG. 7 is a circuit diagram of a pixel circuit according to
a second embodiment of the present invention;
[0021] FIGS. 8A and 8B are graphs of the current flowing through an
organic LED according to the second embodiment after emitting light
for predetermined time;
[0022] FIG. 9 is a circuit diagram of a pixel circuit according to
a third embodiment of the present invention;
[0023] FIG. 10 is a graph of a result of measurement of
fluctuations in the threshold voltage of the TFT in which amorphous
silicon is used for a semiconductor layer; and
[0024] FIG. 11 is a graph of voltage-current characteristic of a
TFT with the current started to flow and the TFT with the current
flowed for long time.
DETAILED DESCRIPTION
[0025] Exemplary embodiments of a thin film transistor in which
fluctuations in current therethrough are suppressed, and image
display apparatus according to the present invention will be
explained in detail with reference to the accompanying drawings.
The present invention is not limited by the embodiments.
[0026] Regarding explanation of the drawings, like reference
numerals designate like parts. The drawings are schematic, and it
should be noted that the relation between the thickness and the
width in the respective layers and the ratio of respective layers
are different from actual products.
[0027] The TFT according to a first embodiment of the present
invention realizes suppression of fluctuations in threshold
voltage, by increasing the distance between the upper end of a gate
electrode and the upper end of an impurity region than the distance
between the upper end of the gate electrode and the upper end of a
channel forming region, to decrease the strength of an electric
field generated between a source electrode and the gate
electrode.
[0028] FIG. 1 is a cross section of the TFT according to the first
embodiment. The TFT 1 includes, on a glass substrate 2, a gate
electrode 3, and a gate insulating film 4 laminated on the gate
electrode 3 so as to cover it. On the gate insulating film 4, the
TFT 1 includes a channel forming region 5c, and a semiconductor
layer 5 having a source region 5a and a drain region 5a, being an
impurity region arranged on a partial region of the channel forming
region 5c and added with impurities. The TFT 1 further includes a
source electrode 6a and a drain electrode 6b, respectively, on the
source region 5a and the drain region 5b, arranged so as to be
electrically connected to the source region 5a and the drain region
5b, respectively, and be away from each other. Further, the TFT 1
has a protective layer 7 arranged so as to cover the source
electrode 6a, the drain electrode 6b, and the channel forming
region 5c. Here, the film thickness of the semiconductor layer 5 in
the region where the source region 5a and the drain region 5b are
arranged is thicker than that of the semiconductor layer 5 in the
region above the gate electrode 3. For the brevity of explanation,
an electrode applied with a voltage higher than that applied to the
other electrode is designated as the drain electrode 6b, and the
other electrode is designated as the source electrode 6a.
[0029] The gate electrode 3 is formed of a conductive material such
as a metal film such as Chromium (Cr), Aluminum (Al) or the like,
alloys thereof, or a transparent material such as Indium Tin Oxide
(ITO). The gate electrode 3 may have a rectangular shape, other
than the trapezoidal shape in cross section, as shown in FIG.
1.
[0030] The gate insulating film 4 is provided between the gate
electrode 3 and the semiconductor layer 5, for electrically
insulating the gate electrode 3 and the semiconductor layer 5, and
is formed of, for example, a transparent insulating film such as
silicon nitride (SiN.sub.x) or silicon oxide (SiO.sub.2), or a
multi-layer film obtained by laminating these films.
[0031] The semiconductor layer 5 having the source region 5a, the
channel forming region 5c, and the drain region 5b is formed of
amorphous silicon. The channel forming region 5c is formed of a
p-type semiconductor, in the case of an n-channel TFT, or an n-type
semiconductor, in the case of a p-channel TFT, but since the TFT 1
is formed of amorphous silicon, doping with impurities may be
omitted. The source region 5a and the drain region 5b are formed of
a semiconductor layer doped with high-density impurities, as
compared with the channel forming region 5c. In the case of the
n-channel TFT, n-type impurities such as phosphorus (P) or arsenic
(As) are doped, and in the case of the p-channel TFT, p-type
impurities such as boron (B) are doped. The TFT 1 according to the
first embodiment will be explained as the n-channel TFT, but the
present invention is not limited to the n-channel TFT, and can be
applied to the p-channel TFT.
[0032] The film thickness of the semiconductor layer 5 in the
region where the source region 5a and the drain region 5b are
arranged is thicker than that of the semiconductor layer 5 in the
region above the gate electrode 3. Therefore, in the TFT 1, the
distance d.sub.1 between the upper end of the source region 5a and
the upper end of the gate electrode 3 is larger than the distance
d.sub.2 between the upper end of the channel forming region 5c and
the upper end of the gate electrode 3.
[0033] The source electrode 6a and the drain electrode 6b are
formed of a conductive material similar to that of the gate
electrode 3. The protective film 7 is formed by using a silicon
nitride film. The protective film 7 is laminated for maintaining
the stable operation of the TFT in the image display area.
[0034] An n-channel is formed in the channel forming region 5c, by
applying a voltage higher than the threshold voltage to the gate
electrode 3. The n-channel conducts electricity between the source
region 5a including the n-type impurities and the drain region 5b.
By applying a predetermined voltage to the drain electrode 6b, the
current flows between the source region 5a and the drain region 5b
via the n-channel. The current flowing between the source region 5a
and the drain region 5b is a drain current. The drain current is
taken in the source electrode 6a or the drain electrode 6b, and
supplied to the outside via a wiring layer connected to the source
electrode 6a and the drain electrode 6b.
[0035] The TFT 1 according to the first embodiment can reduce
fluctuations in the threshold voltage, as compared with the TFT
having the conventional configuration, even when the current has
flowed for long time.
[0036] FIG. 2 is a graph of a result of measurement of fluctuations
in threshold voltage of the TFT 1 according to the first
embodiment. The measurement is conducted for the TFT 1 according to
the first embodiment, as well as the TFT according to a
conventional technology. In the TFT 1 according to the first
embodiment, d.sub.1 is made larger than d.sub.2, by setting the
film thickness of the semiconductor layer 5 in the region where the
source region 5a and the drain region 5b are arranged to 200
nanometers, and the film thickness of the semiconductor layer 5 in
the region above the gate electrode 3 to 100 nanometers. On the
other hand, in the conventional TFT, the film thickness of the
semiconductor layer is 50 nanometers, regardless of the region, and
d.sub.1 and d.sub.2 are equal. This measurement is performed for
measuring the fluctuations in the threshold voltage by maintaining
the state that a voltage of 10 volts is applied to the gate
electrode 3 and the drain electrode 6b for predetermined time.
[0037] As shown in FIG. 2, the fluctuation in the threshold voltage
of the TFT 1 according to the first embodiment is smaller than that
of the conventional TFT, in the whole measurement time. Therefore,
the TFT 1 according to the first embodiment can suppress
fluctuations in the current flowing through the TFT, as compared
with the conventional TFT, even when the TFT is operated for long
time.
[0038] The reason of reduction in the fluctuations in the threshold
voltage in the TFT according to the first embodiment will be
described below. Such a phenomenon that electrons affected by the
electric field generated between the gate electrode 3 and the
source electrode 6a jump across the interface between the gate
insulating film 4 and the semiconductor layer 5 and enter into the
gate insulating film 4 may occur in the TFT. Though not clear at
present, it can be considered that this phenomenon affects the
fluctuations in the threshold voltage. It is presumed that
electrons entering into the gate insulating film 4 have negative
fixed charge, and accompanying this, the threshold voltage
fluctuates in the positive direction. It is considered that the
field strength between the gate electrode 3 and the source
electrode 6a is inversely proportional to the distance between the
gate electrode 3 and the source electrode 6a.
[0039] Here, since d.sub.1 is larger than d.sub.2 in the
configuration of the TFT 1 according to the first embodiment, the
distance between the gate electrode 3 and the source electrode 6a
becomes larger than that of the conventional TFT. Therefore, it can
be considered that the field strength between the gate electrode 3
and the source electrode 6a becomes weak in the TFT 1, and the
influence of electric field with respect to the electrons is
reduced, thereby enabling suppression of fluctuations in the
threshold voltage.
[0040] When operating in the saturated region, the TFT according to
the first embodiment can suppress fluctuations in the threshold
voltage. The reason thereof will be explained below. Here, when the
voltage applied to the drain electrode, that is, the drain voltage
is relatively low, the moving speed of electrons in the n-channel
increases, in proportion to the field strength, and as a result,
the drain current increases linearly, according to the drain
voltage. This operation region is referred to as a linear region.
When the drain voltage is gradually increased, the drain current
does not increase and is saturated, even when the drain voltage is
high. This region is referred to as a saturated region.
[0041] FIG. 3A is a graph of a result of measurement of
fluctuations in the threshold voltage when the TFT according to the
first embodiment operates in a linear region; and FIG. 3B is a
graph of a result of measurement of fluctuations in the threshold
voltage when the TFT according to the first embodiment operates in
a saturated region. The measurement is performed for a plurality of
TFTs manufactured under the same condition.
[0042] As shown in FIG. 3A, when the TFT 1 operates in the linear
region for 10.sup.6 seconds, the threshold voltage fluctuates to
V.sub.1. On the other hand, as shown in FIG. 3B, when the TFT 1
operates in the saturated region for 10.sup.6 seconds, the
threshold voltage only fluctuates to V.sub.2 (<V.sub.1).
Therefore, in the TFT 1, fluctuations in the threshold voltage are
reduced when the TFT 1 operates in the saturated region, than when
it operates in the linear region. Therefore, when it is assumed
that a point in time when the threshold voltage fluctuates by a
certain value is the end of the life span of the TFT, the life span
of the TFT according to the first embodiment can be further
extended by operating in the saturated region.
[0043] The reason for further suppression in fluctuations in the
threshold voltage by operating the TFT according to the first
embodiment in the saturated region can be presumed as follows. It
is presumed that the phenomenon in which the electrons supplied
from the source region enter into the gate insulating film 4 is
affected by the magnitude correlation between the gate-source
voltage and the source to drain voltage. In the saturated region,
since the voltage applied to the source electrode is lower than
that applied to the drain electrode and the gate electrode, and the
voltage applied to the drain electrode is high, the difference
between the source to drain voltage and the gate-source voltage is
reduced. Therefore, it is construed that the rate of electrons that
do not enter into the gate insulating film 4 and pass through the
channel to move to the drain electrode becomes high. Therefore, it
is presumed that the TFT according to the first embodiment can
further suppress the fluctuations in the threshold voltage by
operating in the saturated region.
[0044] The TFT according to the first embodiment can suppress the
fluctuations in the threshold voltage by operating in the saturated
region, even when it is used in the ON state continuously.
Therefore, even when being used continuously, the TFT according to
the first embodiment can reduce a decrease in the current flowing
through the TFT, and hence can be used for a longer period of time
than the conventional TFT.
[0045] In the TFT according to the first embodiment, the channel
length can be made short. FIG. 4 is a graph of fluctuation
characteristics of the threshold voltage of TFTs having different
channel length in the TFT according to the first embodiment. Curve
11 indicates the fluctuation characteristic of the threshold
voltage of the TFT having a channel length of 4.5 micrometers, and
curve I.sub.2 indicates the fluctuation characteristic of the
threshold voltage of the TFT having a channel length of 6.5
micrometers. As shown in FIG. 4, there is not much difference
between the curve I.sub.1 and the curve I.sub.2, and hence it is
considered that the difference in the channel length does not
affect fluctuations in the threshold voltage. Therefore, the
channel can be made short in the TFT according to the first
embodiment, thereby realizing miniaturization of the TFT.
[0046] As the TFT according to the first embodiment, a TFT in which
the film thickness of the semiconductor layer 5 in the region where
the source region 5a and the drain region 5b are arranged is set to
200 nanometers, and the film thickness of the semiconductor layer 5
in the region above the gate electrode 3 is set to 100 nanometers
is explained. However, in the film thickness of the semiconductor
layer 5, a difference of about 15% occurs, and specifically, the
film thickness of the semiconductor layer 5 in the region where the
source region 5a and the drain region 5b are arranged becomes from
170 to 230 nanometers inclusive. Further, the film thickness of the
semiconductor layer 5 in the region above the gate electrode 3
becomes from 85 to 115 nanometers inclusive. The film thickness of
the semiconductor layer 5 in the respective regions is not limited
thereto. If the film thickness of the semiconductor layer 5 in the
region where the source region 5a and the drain region 5b are
arranged is a thickness in which impurities diffuse sufficiently,
it is enough, and can be made thinner or thicker with respect to
200 nanometers, so long as it satisfies this condition. It is
necessary to suppress the film thickness of the semiconductor layer
5 in the region above the gate electrode 3 to a thickness in which
leakage does not occur between the source region 5a and the drain
region 5b. On the other hand, it is necessary to increase d.sub.1
in order to weaken the field strength between the gate electrode 3
and the source electrode 6a. Therefore, the semiconductor layer 5
in the TFT according to the first embodiment has a stepped
configuration in the region above the gate electrode 3, and the
region where the source region 5a and the drain region 5b are
arranged. The film thickness of the semiconductor layer 5 in the
region above the gate electrode 3 may be set to, for example, 50
nanometers, as a thickness satisfying the condition that a leakage
does not occur between the source region 5a and the drain region
5b.
[0047] As the TFT according to the first embodiment, the TFT 1 in
which the gate insulating film 4 and the semiconductor layer 5 are
provided between the gate electrode 3 and the source electrode 6a
and between the gate electrode 3 and the drain electrode 6b, is
explained, but another layer may be further provided, so long as
the configuration is such that d.sub.1 is larger than d.sub.2.
[0048] In the TFT according to the first embodiment, d.sub.1 is
made larger than d.sub.2 by providing a difference in the film
thickness of the semiconductor layer 5, or d.sub.1 may be made
larger than d.sub.2 as shown in FIG. 5, by making the film
thickness of the gate insulating film 14 corresponding to a region
14a and a region 14b larger than that of the gate insulating film
14 corresponding to a region 14c.
[0049] FIGS. 6A to 6D are schematics for illustrating a method of
manufacturing the TFT according to the first embodiment. By
adopting the method of manufacturing the TFT 1, as shown in FIGS.
6A to 6D, the manufacturing cost can be reduced.
[0050] FIG. 6A is a schematic of a step of forming the gate
electrode 3 on the glass substrate 2. The gate electrode 3 is
formed by etching using a mask pattern having a predetermined
opening. A case of using a taper etching method is depicted, in
which the cross section becomes trapezoidal, but an etching method
in which the cross section becomes rectangular may be used.
[0051] FIG. 6B is a schematic of a step of forming the gate
insulating film 4, an amorphous silicon layer 8, a high-density
n-type amorphous silicon layer 9, and a metal thin film layer 6 on
the gate electrode 3. The amorphous silicon layer 8 is for forming
the channel forming region 5c, and the high-density n-type
amorphous silicon layer 9 is for forming the source region 5a and
the drain region 5b at the subsequent step. The metal thin film
layer 6 is for forming the source electrode 6a and the drain
electrode 6b at the subsequent step.
[0052] FIG. 6C is a schematic of a step of forming the source
electrode 6a and the drain electrode 6b by the etching process. At
this step, the metal thin film layer 6 in the region other than the
region corresponding to the source electrode 6a and the drain
region 6b, and the high-density n-type amorphous silicon layer 9 in
the region other than the region corresponding to the source region
5a and the drain region 5b are removed. By setting a large etching
quantity, a part of the amorphous silicon layer 8 in the region
above the gate electrode 3 is also removed. After the etching step
in FIG. 6C, the protective film 7 is formed as shown in FIG.
6D.
[0053] According to the method explained above, the TFT according
to the first embodiment can be produced. Removal of the metal thin
film layer 6 and the high-density n-type amorphous silicon layer 9,
and reduction in film thickness of the amorphous silicon layer 8 in
the region above the gate electrode 3 become possible only by one
etching process, thereby enabling reduction in the production
cost.
[0054] The image display apparatus according to a second embodiment
will be explained next. The image display apparatus according to
the second embodiment uses the TFT according to the first
embodiment as the driver element, and includes a light emitting
element in which the value of the current flowing therethrough is
controlled by the driver element.
[0055] FIG. 7 is a circuit diagram of a pixel circuit according to
a second embodiment of the present invention. The image display
apparatus according to the second embodiment is formed by arranging
a pixel circuit 21 in a matrix. The pixel circuit 21 includes an
organic LED 22 being a light emitting element, a TFT 23 being a
driver element, a data line 24 for supplying brightness data to the
pixel circuit in the form of voltage, and a TFT 25 being a
switching element. The pixel circuit 21 further includes a
capacitor 27 that holds the voltage supplied from the data line 24,
and a scan line 26 that controls the driven state of the TFT
25.
[0056] The organic LED 22 functions as the light emitting element,
and emits light with brightness corresponding to the magnitude of
the current flowing therethrough. The organic LED 22 is connected
to one of the source and drain electrodes of the TFT 23 on one
side, and to a constant power line Vdd on the other side.
[0057] The TFT 23 functions as the driver element, and controls
light emission of the organic LED 22 and the intensity at the time
of light emission, by controlling the current flowing through the
organic LED 22. The TFT 23 and the organic LED 22 are connected in
series, and when the current flows through the TFT 23, the current
equal to the current flowing through the TFT 23 flows in the
organic LED 22.
[0058] The source electrode of the TFT 23 is connected to the
ground, and the drain electrode thereof is connected to the
constant power line Vdd via the organic LED 22. Sufficiently high
voltage is supplied to the drain electrode of the TFT 23 by the
constant power line Vdd, and the TFT 23 regulates the value of the
current flowing through the channel by the voltage supplied to the
gate electrode. Therefore, the TFT 23 becomes the ON state in the
saturated region. The TFT 23 has the configuration described in the
first embodiment, and has a characteristic such that even when the
ON state is maintained for long time in the saturated region,
fluctuations in the threshold voltage are reduced, thereby reducing
a decrease in the value of the current flowing in the channel.
[0059] The TFT 25 functions as the switching element, and controls
supply of voltage from the data line 24 to the gate electrode of
the TFT 23. When being in the ON state, the TFT 25 conducts
electricity between the data line 24 and the gate electrode of the
TFT 23. As a result, the data line 24 can supply predetermined
voltage to the gate electrode of the TFT 23. Further, the scan line
26 controls the driven state of the TFT 25, and by setting the scan
line 26 to the high level, the TFT 25 becomes the ON state, and by
setting the scan line 26 to the low level, the TFT 25 becomes the
OFF state.
[0060] The operation of the pixel circuit 21 until the organic LED
22 emits light will be explained below. When the scan line 26
becomes high level and the TFT 25 becomes the ON state, voltage is
supplied from the data line 24 to the gate electrode of the TFT 23.
When the scan line 26 is set to the low level in order to set the
TFT 25 to the OFF state, the data line 24 is electrically cut off
from the TFT 23, but the voltage at the gate electrode of the TFT
23 is held stably by the capacitor 27. The current flowing through
the TFT 23 and the organic LED 22 has a value corresponding to the
gate-source voltage of the TFT 23, and the organic LED 22 emits
light with brightness corresponding to the current value.
[0061] FIGS. 8A and 8B are graphs of the current flowing through an
organic LED 22 according to the second embodiment after emitting
light for predetermined time. FIG. 8A depicts the value of the
current flowing through the organic LED 22 for each frame, when the
organic LED 22 included in the pixel circuit 21 starts to emit
light, and FIG. 8B depicts the value of the current flowing through
the organic LED 22 for each frame, after the organic LED 22 has
continuously emitted light for 30,000 hours.
[0062] As shown in FIG. 8A, at the initial stage when the organic
LED 22 starts to emit light, a current of about 5.5 microamperes
flows through the organic LED 22 for each frame, over about 0.8
microsecond. On the other hand, after the organic LED 22 has
emitted light continuously for 30,000 hours, as shown in FIG. 8B, a
current of about 4.3 microamperes flows through the organic LED 22
for each frame, over about 0.8 microsecond, which means a reduction
of only about 22% of the current flowing therethrough for each
frame, as compared with the initial stage of light emission. When
it is assumed that the end of the life span of the image display
apparatus is when the emission brightness of the organic LED 22
decreases to 50% of that of the initial stage of light emission, in
the image display apparatus according to the second embodiment,
even when the organic LED 22 emits light continuously for 30,000
hours, the current flowing through the organic LED 22 shows only
22% reduction, as compared with the initial stage of light
emission. Therefore, it is considered that the life span of the
image display apparatus according to the second embodiment can be
extended as compared with the conventional image display
apparatus.
[0063] The image display apparatus according to the second
embodiment can reduce a decrease in the value of the current
flowing through the organic LED 22, by using the TFT according to
the first embodiment as the driver element, thereby enabling
suppression of fluctuations in the emission brightness of the
organic LED 22 for long time. Therefore, the image display
apparatus according to the second embodiment can perform
high-quality image display, keeping the uniformity in the display
brightness over long time, thereby enabling suppression of
deterioration in the image display.
[0064] The image display apparatus according to the second
embodiment is explained for a case when the organic LED 22 is used
as the light emitting element. However, since it is sufficient to
use a light emitting element that emits light with brightness
corresponding to the magnitude of the flowing current, for example,
an inorganic LED or a light emitting diode can be used other than
the organic LED.
[0065] In the image display apparatus according to the second
embodiment, the specific configuration of the TFT 25 being the
switching element is not specifically explained, but the TFT
according to the first embodiment may be used also for the TFT 25.
The TFT according to the first embodiment has not only the
characteristic of suppressing fluctuations in the threshold
voltage, but also the characteristic of the conventional TFT.
Therefore, the TFT according to the first embodiment can be used
not only for the one that conducts electricity continuously, but
also for a voltage driver element such as the switching
element.
[0066] The image display apparatus according to a third embodiment
will be explained next. In the second embodiment, the TFT according
to the first embodiment is used as the driver element, but in the
image display apparatus according to the third embodiment, the TFT
according to the first embodiment is used as a current determining
element that determines the value of the current flowing through
the driver element.
[0067] FIGS. 8A and 8B are graphs of the current flowing through an
organic electro-luminescence according to the second embodiment
after emitting light for predetermined time. The image display
apparatus according to the third embodiment is formed by arranging
a pixel circuit 31 in a matrix. The pixel circuit 31 includes an
organic LED 32 being the light emitting element, and a TFT 33 being
the driver element. Further, the pixel circuit 31 includes a data
line 34 for supplying a predetermined voltage to the source
electrode of a TFT 35, the TFT 35 being the current determining
element, a scan line 36 that applies a predetermined voltage to the
gate electrode of the TFT 35, and a capacitor 37 that converts the
current flowing through the TFT 35 into voltage and holds the
voltage. The pixel circuit 31 further includes a TFT 40 that
controls short circuit between the gate electrode and the drain
electrode of the TFT 33, and a TFT 41 that controls electrical
conduction between the source electrode of the TFT 33 and the
ground.
[0068] The TFT 35 has the configuration described in the first
embodiment, and has characteristics of suppressing fluctuations in
the threshold voltage and suppressing a decrease in the value of
the current flowing through the TFT 35, even when the ON state
thereof is kept in the saturated region for long time.
[0069] The TFT 35 functions as the current determining element, and
determines the value of the current flowing through the TFT 33
based on the voltage applied to the gate electrode of the TFT 35,
by operating in the saturated region at the time of voltage write.
The value of the current flowing through the TFT 35 is a value
determined corresponding to the brightness to be realized by the
organic LED 32. In order to allow the current of this value to flow
through the TFT 35, the scan line 36 applies the voltage
corresponding to the current value to the gate electrode of the TFT
35. The current flowing through the TFT 35 is supplied as the
gate-source voltage of the TFT 35, after having been converted into
voltage by the capacitor 37, and becomes the voltage value when the
current corresponding to the emission brightness flows through the
organic LED 32.
[0070] The organic LED 32 and the TFT 23 function in the same
manner as the organic LED 22 and the TFT 33 in the second
embodiment. The TFT 40 has a function of detecting the threshold
voltage of the TFT 33, by short-circuiting between the gate
electrode and the drain electrode of the TFT 33. The TFT 41 has a
function of connecting the source electrode of the TFT 33 and the
ground by becoming the ON state, so as to allow the current to flow
through the TFT 33.
[0071] The operation of the pixel circuit 31 until the organic LED
32 emits light will be explained. When the TFT 40 becomes the ON
state to short-circuit between the gate electrode and the drain
electrode of the TFT 33, the pixel circuit 31 detects the threshold
voltage of the TFT 33. Thereafter, since the scan line 36 applies a
predetermined voltage to the gate electrode of the TFT 35, the TFT
35 becomes the ON state in the saturated region, and allows the
current of a value determined based on the applied voltage to flow.
The value of the current flowing through the TFT 35 is a value
corresponding to the brightness to be realized by the organic LED
32. The capacitor 37 converts the current having flowed to the TFT
35 into voltage, holds the voltage, and supplies the held voltage
to the TFT 33 as the gate-source voltage.
[0072] The TFT 41 then conducts electricity to connect the source
electrode of the TFT 33 and the ground. As a result, the current
flows through the TFT 33 and the organic LED 32, and the organic
LED 32 emits light. The current flowing through the TFT 33 and the
organic LED 32 has a value corresponding to the gate-source voltage
of the TFT 33, that is, the current having flowed to the TFT
35.
[0073] As described above, the TFT 35 being the current determining
element determines the value of the current flowing through the TFT
33 based on the voltage applied to the gate electrode of the TFT
35. When fluctuations in the threshold voltage occur in the TFT 35,
the current flowing through the TFT 33 fluctuates, thereby making
the emission brightness of the organic LED 32 nonuniform. However,
in the image display apparatus according to the third embodiment,
the TFT according to the first embodiment is used as the current
determining element, thereby suppressing fluctuations in the
threshold voltage of the current determining element. Therefore,
the current of a predetermined value flows to the TFT 33 without
fluctuations, and hence fluctuations in the emission brightness due
to the fluctuations in the current flowing through the organic LED
32 can be reduced, thereby realizing the image display apparatus
that can perform high-quality image display for long time.
[0074] In the image display apparatus according to the third
embodiment, the specific configuration of the TFT 40 and the TFT
41, being a TFT other than the current determining element, is not
particularly explained, but the TFT according to the first
embodiment may be used for the TFT 40 and the TFT 41. As in the
image display apparatus according to the second embodiment, the TFT
according to the first embodiment may be used as the driver
element.
[0075] Although the invention has been described with respect to a
specific embodiment for a complete and clear disclosure, the
appended claims are not to be thus limited but are to be construed
as embodying all modifications and alternative constructions that
may occur to one skilled in the art which fairly fall within the
basic teaching herein set forth.
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